Fix minor things.

Author: drupol

src/Modifier/ModifierInterface.php

@@ -15,10 +15,10 @@ interface ModifierInterface
      * Modify the tree.
      *
      * @param NodeInterface $tree
-     *   The original tree
+     *   The original tree.
      *
-     * @return NodeInterface
-     *   A new tree
+     * @return \drupol\phptree\Node\AttributeNodeInterface|\drupol\phptree\Node\MerkleNodeInterface|\drupol\phptree\Node\ValueNodeInterface|NodeInterface
+     *   A new tree.
      */
     public function modify(NodeInterface $tree): NodeInterface;
 }

src/Node/MerkleNode.php

@@ -79,7 +79,7 @@ private function doHash(MerkleNodeInterface $node): string
     {
         // If node is a leaf, then compute its hash from its value.
         if (true === $node->isLeaf()) {
-            return $this->hasher->hash($node->getValue());
+            return $this->hasher->hash((string) $node->getValue());
         }
 
         $hash = '';